The PWE3 technology is used for emulating the basic features of the services such as Asynchronous Transfer Mode (ATM), Frame Relay (FR), Ethernet, low-speed Time Division Multiplexing (TDM) circuit, and Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH) in a Packet Switched Network (PSN). The functions of a Pseudo Wire (PW) include: encapsulating the Protocol Data Units (PDUs) of a specific service at the ingress, and bearing such PDUs on the path or tunnel between the ingress and egress, managing the timing and sequence of the PDUs, and providing the functions required for emulating the actions and features of such services as far as possible. As for users, a PW is only an independent link or circuit.
In a PWE3 system, the Customer Edge (CE) equipment is connected to the Provider Edge (PE) equipment through an Attachment Circuit (AC). Multiple PWs used for connecting two ACs are carried in a PSN tunnel between the two PEs of the two ACs. The AC is a physical circuit or virtual circuit. One AC may be a Data Link Connection Identifier (DLCI) of a Frame Relay (FR), a Virtual Path Identifier (VPI)/Virtual Channel Identifier (VCI) of an ATM, a port of an Ethernet, a Virtual Local Area Network (VLAN), a Point-to-Point Protocol (PPP) connection on a physical interface, or a Label Switched Path (LSP) of Multi-Protocol Label Switching (MPLS).
The BFD is a mechanism for quickly detecting whether a forwarding path between a pair of forwarding equipments is available. It provides a failure detection mechanism characterized by low cost and short detection periods between two adjacent systems, and covers detection of interfaces, data links and forwarding equipments. The BFD is similar to a “Hello” protocol. After a BFD session is set up between two systems that need to be detected, both parties send a BFD packet to the peer periodically, and detect the arrival of the packet from the peer periodically on the link where the BFD is applied. If no BFD packet is received from the peer within a specific interval, the link is regarded as faulty, thus detecting the link fault quickly.
In a PWE3 system, the PW is bidirectional. The PW works (in the “UP” status) only after the PWs of the same Pseudo Wire Identifier (PWID) are set up in both the ingress PE direction and the egress PE direction, and can constitute a BFD path properly. Both sides of the PW send BFD packets reciprocally. If a packet identified “down” is received from the peer or if no packet is received from the peer within a preset interval, the PW is regarded as faulty.
In the conventional art, a BFD session of a PW is set up through LSP ping auto negotiation. As defined in draft-ietf-bfd-mpls-03, the process of negotiating a BFD discriminator through an LSP ping is: The ingress PE sends its own discriminator through the LSP ping of the PW, and obtains the discriminator of the peer from the LSP ping response packet of the egress PE. The main process is as follows:
1. When configuring the PW, configuring the BFD-related parameters (such as minimum interval of sending packets, minimum interval of receiving packets), and Virtual Circuit Connection Verification (VCCV) capability parameters of the PW, including Connection Channel (CC) parameters (such as PW control word, alert label), and Connection Verification (CV) parameters (such as BFD capability).
2. Negotiating the BFD discriminator through the LSP ping after the PW is set up and the forwarding items are determined.
3. Setting up a BFD session after the BFD discriminator is negotiated successfully through the LSP ping.
4. Detecting whether the PW forwarding process fails according to the BFD session.
As shown in FIG. 1, the format of the Type Length Value (TLV) of the BFD discriminator carried in the LSP ping packet includes: Type, Length, and BFD discriminator.
The BFD session discriminator of the PW in the PWE3 system in the conventional art is negotiated through the LSP ping. When keeping the BFD configuration status consistent between the two equipments on the two ends of the PW, the system needs to consider the time sequence relation between the LSP ping and the PW connection/disconnection (up/down), thus increasing the complexity of system processing. The interworking routers have to support LSP ping, which leads to high consumption of the CPU resources of the equipment. The PEs at both sides of the PW are unable to perceive the BFD configuration status of the PW in time. Moreover, when the BFD configuration status of the two equipments at both sides of the PW is not symmetric, namely, when the equipment at one side of the PW cancels the BFD capability or adds the BFD capability during operation, the PW has to be disconnected, renegotiated and connected again, thus leading to traffic interruption and reducing efficiency of maintaining the PW.